In a case where a passenger seated on a seat for a vehicle does not fasten a seat belt, the passenger may be warned by a buzzer and the like so as to fasten the seat belt. Then, a load detecting device for detecting a load of the passenger seated on the seat is provided at the seat. When the load detected by the load detecting device is greater than a predetermined threshold value, it is determined that the passenger is seated on the seat. On the other hand, when the load detected by the load detecting device is equal to or smaller than the threshold value, it is determined that the passenger is not seated on the seat.
Load sensors constituting the load detecting device, which are disclosed in JPH09-207638A, for example, are arranged at four portions, respectively, between a seat-side fixing member for supporting a seat for a vehicle and a floor-side fixing member for fixing the seat-side fixing member to a floor of the vehicle, while having a predetermined distance therebetween in a longitudinal direction and a width direction of the vehicle. Even when the passenger is seated in any posture on the seat, the load of the passenger is accurately detectable on a basis of sum of outputs from the four load sensors.
As illustrated in FIG. 8, in a case where the load detected by the load detecting device is equal to or greater than 18 kg, for example, and such detection elapses for 0.5 sec, it is determined that the passenger is seated on the seat (passenger seated state). At this time, when the passenger does not fasten the seat belt, a warning member warns the passenger to fasten the seat belt. In a case where the load detected by the load detecting device is smaller than 5 kg, for example, and such detection elapses for 0.5 sec, it is determined that no passenger is seated on the seat (no passenger seated state) and the warning member is prohibited to operate.
In order to reduce a cost of manufacturing, the number of load sensors constituting the load detecting device is reduced to two by which the load of the passenger seated on the seat is detected. However, in a case where the two load sensors are provided at the rear-left and rear-right portions of the seat, the load detected by the load detecting device fluctuates, depending on how the load is applied to the seat, which may prevent an accurate determination of seat occupancy.
For example, whether the passenger is seated at a forward portion of a seat cushion or seated back on the seat cushion leads to a difference in the load detected by the load detecting device. As illustrated in a transition state of FIG. 8, the determination is frequently switched between the passenger seated state and the no passenger seated state.
The frequent switching between the passenger seated state and the no passenger seated state may occur not only depending on how the passenger is seated on the seat but also when the vehicle is driven with luggage placed on the seat (passenger seat), for example. That is, in a case where luggage is placed on the seat in a normal condition, the load detected by the load detecting device is prevented from exceeding a threshold value (i.e., 18 kg) for determining the passenger seated on the seat. Thus, the warning for the passenger who does not fasten the seat belt is not activated. However, when the vehicle is suddenly accelerated and the acceleration is applied to the seat, for example, the load of luggage is applied to the rear portion of the seat. Then, the load detected by the load detecting device may exceed the threshold value for determining the passenger seated on the seat. In such case, nevertheless the passenger is not seated on the seat, the warning for the passenger to fasten the seat belt is mistakenly activated. Afterwards, when the acceleration acting on the vehicle decreases, the load detected by the load detecting device becomes smaller than the threshold value so that the warning for the passenger to fasten the seat belt is stopped. When the acceleration again acts on the seat, the warning for the passenger to fasten the seat belt is again activated.
Accordingly, even when the luggage is placed on the passenger seat, the determination is frequently switched between the passenger seated state and the no passenger seated state depending on a driving state of the vehicle. The warning for the passenger to fasten the seat belt is repeatedly activated, which results in an uncomfortable feeling of the passenger such as a driver, and the like.
A need thus exists for a seat belt warning device which is not susceptible to the drawback mentioned above.